As has been discussed in U.S. Pat. No. 4,006,952, issued Feb. 8, 1977 to William E. Puckett, a need exists for a battery charging system which can be mounted in a service vehicle to start stalled vehicles using electrical energy supplied by the battery of the service vehicle. The apparatus disclosed in that patent includes a reel which is disposed in a casing mounted on the service vehicle and upon which jump cables can be wound. Circuitry is supplied for completing an electrical connection between the batteries of the two vehicles following connection of battery clamps, on the ends of the cables, to the battery of the stalled vehicle. Such an apparatus, because of its mobility, provides the useful service of being on call to any motorist who is caught in a situation in which he cannot start his automobile because, for one reason or another, his battery has become discharged.